There is an increasing need, both on the part of the electric utility industry and of end-users, to improve and maintain power quality. The reasons for this are many, but generally include the direct cost of billable power delivered to a user. Hence, where current and voltage are in phase with each other (unity power, i.e. having a power factor of 1, pf 1) the maximum amount of power is delivered and hence, can be billed. Further, and perhaps to some extent more importantly, the secondary costs of power are becoming more dominant. By secondary costs, are meant equipment maintenance problems resulting in reduced life or efficiencies when unity power is not achieved. One example of this is overheating and/or inefficient operation of motors as current lags voltage.
Moreover, this problem is even more pronounced of late as a result of recent regulatory changes which allow electric power users to purchase power from distant producers of electricity, rather than the local utility company. As such, due to the potential for a great number of users connecting to, placing power on, or removing power from the electric grid, the quality of power is not only apt to have greater swings to it, it is likely to happen much more frequently.
Solutions for correcting power factor in order to bring the phase angle closer to unity typically incorporate what are commonly referred to as VAR generator systems. These have taken many forms, the most prevalent being capacitor banks, but all attempt to adjust the timing and hence phase of the voltage in relation to the current in order to bring it in line with current. Power, in an alternating current system, is based upon the value of the voltage multiplied by the value of the current and of the cosine of the phase angle between them (P=VIcos.theta.). Accordingly, where current and voltage are in phase unity is achieved and the phase angle is zero and hence the cosine of zero degrees is one and, therefore, power approaches its maximum amount (the voltage times the current (P=VI)).
It is well known that loads on the system, as well as transmission lines themselves, are generally inductive (and sometimes reactive) in nature. Therefore, the current will lag the voltage by some amount theta (.theta.). Therefore, as a practical matter, the power factor is less than one. Since consumers of electricity are metered upon current consumed, rather than the mathematical product of the two multiplied by the phase angle, consumers are, in effect, charged at a rate equal to a unity of one, but in fact receive less power. While utility companies attempt to provide power at unity, it is extremely expensive and difficult to do, despite the fact that this also indirectly affects the utilities ability to effectively meter and control their grid, thereby depriving them of potential revenue or to adjust the size of the grid to compensate for losses. Moreover, equipment, particularly industrial equipment, will consume more current and hence more power when the power factor is less than 1. This in turn requires the utility to produce, distribute and control more power which is costly. Hence, there is an incentive for utilities and consumers to produce and obtain optimum power, i.e. voltage and current having a power factor of 1.
As mentioned, the typical solution is to add capacitor banks either at substations or to the distribution lines in the grid. Further, some large users attempt to also add capacitor banks at the individual point of consumption. While this will of course improve the voltage at the end location, switching of capacitor banks in response to varying changes in the system, generates voltage transients that are frequently more objectionable than the low voltage/low power at the end location. These voltage transients cause problems themselves and the advent of computers, processors and the like, has made these disturbances more objectionable. Further, losses of computer systems and sub-systems cause even more problems when they are adversely affected.
As previously indicated, a number of methods have been tried to overcome these problems. One such solution may be found in U.S. Pat. No. 3,573,549 entitled "Electrical System Including Capacitors", issued Apr. 1, 1971 to Wolf. There, it is taught that capacitance in a load circuit is decreased in response to the changes in the applied voltage wherein capacitors are connected parallel to each other which are then connected in series with the load, and wherein a second bank of capacitors is connected in parallel with the first bank such that the addition or deletion of the second bank boosts voltage and, hence, the KVARs supplied to the circuit are adjusted.
A schematic diagram of this may be seen in FIG. 1, in which, typically, there is a capacitor bank VAR system showing generally as (10). This VAR system (10) is comprised of a power source which may be an electrical generation facility, a sub-station or the like (12). The system impedance is shown schematically in box (14) and represents the impedance of the system as a function of transmission equipment, lines, length of lines and the like. Similarly, the load (16) is shown. Connected serially between the power source (12) and the load (16) are capacitor banks A and B (18), (20) respectively. Capacitor bank A is comprised of capacitors (22), (24) and (26), while similarly, capacitor bank B is comprised of capacitors (23), (25) and (27). As can be seen, steady state current flows through capacitor bank A (18) and by closure of bank switch (28) capacitor bank B (20) is placed in parallel with capacitor bank A (18).
Accordingly, the customary intent and solution of VAR systems is to place a varying number of electrically parallel capacitors in series as a bank in order to achieve a unity power factor or as close thereto as possible. In this fashion, it can be seen that phase control is obtained by switching the entire bank on or off, or multiples of banks on or off in one operation in an attempt to retard the voltage with respect to the current. In practice, it is readily known that finer resolution is obtained by making multiple banks of smaller sizes. However, this is obviously more expensive since each bank must be separately switched while also requiring additional space. Similarly, with increased numbers of capacitors for finer control the likelihood and frequency of voltage transients and, hence, damage to down-line equipment is greatly increased.
Other examples of automated VAR-type systems may be found in U.S. Pat. No. 5,422,561 entitled "Automated Voltage And VAR Control In Power Transmission And Distribution Networks", issued Jun. 6, 1995 to Williams, et al. There, an elaborate algorithm-based scheme is utilized to again simply switch capacitors in and out of the network. Another VAR-type of system may be found in U.S. Pat. No. 4,769,587 entitled "Power Factor Control With Overvoltage And Undervoltage Override Control In A Capacitor Control System For A Distribution System", issued Sep. 6, 1988 to Pettigrew. This patent again teaches the use of switching in or out a plurality of capacitors in the capacitor bank with method and apparatus to modify the operating characteristics of the system depending on load conditions, i.e., light versus heavy loads.
U.S. Pat. No. 4,719,402 entitled "VAR Generator System With Minimal Stand-by Losses", issued Jan. 12, 1988 to Brennen, et al, utilizes capacitor banks wherein thyristor switches are utilized to add or delete the reactive components such as the capacitor bank or an inductor.
U.S. Pat. No. 4,684,875 entitled "Power Conditioning System And Apparatus", issued Aug. 4, 1987 to Powell, utilizes a synthesizer network to develop a "stiffness" to suddenly lagging phase angles and the like, again with capacitor banks.
U.S. Pat. No. 4,645,997 entitled "Transient Free Solid State Automatic Power Factor Correction", issued Feb. 4, 1987 to Whited, utilizes capacitors hooked up in a Delta-type configuration through use of optically isolated semi-conductor devices.
U.S. Pat. No. 4,482,857 entitled "Close Loop Power Factor Control For Drilling Rigs, issued Nov. 13, 1984 to Porche, et al., utilizes an over-excited generator in order to provide reactive power to effect power factor correction.
U.S. Pat. No. 4,307,331 entitled "Hybrid Switched-Capacitor Controlled-Inductor Static VAR Generator In Control Apparatus, issued Dec. 22, 1981 to Gyugyi, utilizes a VAR detector/controller in conjunction with parallel capacitors to switch in a bank of capacitors in order to effect phase angle changes.
U.S. Pat. No. 4,275,346 entitled "Static VAR System With Means For Correcting For Negative Phase Shift", issued Jun. 23, 1981 to Kelley, Jr., discloses the use of phase control switches with a capacitor bank.
Moreover, the larger the system or the finer the control necessary or desired, the more expensive and larger in complexity the control system becomes.